Certain semiconductor devices, e.g., magnetic random access memory (MRAM) devices, use magnetic memory cells to store information. Each magnetic memory cell typically comprises a submicron piece of magnetic material, e.g., having the dimensions of 300 nanometers (nm) by 600 nm in area and five nm thick.
Information is stored in such semiconductor devices as an orientation of the magnetization of a free layer in the magnetic memory cell as compared to an orientation of the magnetization of a fixed (e.g., reference) layer in the memory cell. The magnetization of the free layer may be oriented parallel or anti-parallel relative to the fixed layer, representing either a logic “1” or a “0.” The orientation of the magnetization of a given layer (fixed or free) may be represented by an arrow pointing either to the left or to the right. When the magnetic memory cell is sitting in a zero applied magnetic field, the magnetization of the magnetic memory cell is stable, pointing either left or right. The application of a magnetic field can switch the magnetization of the free layer from left to right, and vice versa, to write information to the magnetic memory cell.
A particular type of magnetic memory cell, a “toggle” magnetic memory cell, employs a free layer comprising two magnetic layers separated by a non-magnetic spacer layer. This multi-layer free layer is typically separated from a fixed layer of the magnetic memory cell by a tunnel barrier, such that one of the multiple layers making up the free layer is adjacent to the tunnel barrier. It is this layer adjacent to the tunnel barrier that primarily determines a resistance of the tunnel junction, which relates to the information stored.
The fixed layer typically comprises one or more pinned layers. The one or more pinned layers can be formed from a single magnetic layer (e.g., a simple pinned layer), or alternatively, from two magnetic layers tightly anti-parallel coupled by a non-magnetic spacer layer (e.g., an anti-parallel (AP) pinned layer). The one or more pinned layers, either simple or AP, have a fixed direction of magnetization corresponding to an anti-ferromagnetic layer to which the one or more pinned layers are pinned.
With these typical toggle magnetic memory cell configurations, however, Neel coupling often causes the one or more pinned layers to disturb the operation of the free layer. Neel coupling, also referred to as “orange peel” coupling, arises from the formation of magnetic poles on the rough magnetic interfaces of the magnetic layers, e.g., between the one or more pinned layers, the free layer and the barrier layer.
The magnetic poles formed induce the directions of magnetization of the magnetic layers on either side of the barrier to line up parallel to each other. One way to reduce Neel coupling is to reduce surface interface roughness of the layers. Reducing the roughness of the layers, however, is difficult under typical manufacturing conditions, and in practice, some residual roughness always remains. Therefore, some amount of Neel coupling is always present.
Neel coupling disadvantageously reduces the write margins of the semiconductor device. Therefore, it would be desirable to reduce, or eliminate, the effects of Neel coupling on the switching of toggle magnetic memory cells.